Method for trimming cermet resistors



United States Patent Cfice 3,457,637 METHOD FOR TRIMMING (IERMET RESISTORS Allan S. Halpern, New York, N.Y., assignor to General Instrument Corporation, Newark, N.J., a crporati0n of New Jersey No Drawing. Filed Feb. 6, 1967, Ser. No. 614,037

-Int. Cl. H01c 17/00 U.S. Cl. 29-620 24 Claims ABSTRACT OF THE DISCLOSURE Reducing and stabilizing the resistance value of cermet resistors by passing a current of appropriate value therethrough, thereby controllably fusing abutting conductive particles to one another.

The present invention relates to a method for reducing and stabilizing the resistance value of a cermet resistor so that said resistance value Will remain at a constant and desired value while the resistor is in use.

The procedure of adjusting the resistance value of a resistor is generally termed trimming. It is necessary because many resistors are so constructed and constituted that as a practical matter it is impossible solely by fundamental fabrication techniques to cause them to have a desired resistance value. This is particularly true of resistors of the type suitable for use in integrated circuits. Such resistors comprise a layer of resistive material which is deposited on an insulating substrate, the resistance of the layer being dependent in part upon the composition thereof and in part upon its physical dimensions-length, width, and thickness. One general category of resistor of this nature which is in widespread use is the so-called cermet resistor. It comprises a large number of discrete particles contained within a matrix. Different types of particles are employed in different types of cermet resistors. The cermet resistor composition in connection with which the present invention is applicable is one in which a large number of discrete particles of a nonoxidizable, noble metal alloy are contained within a glass matrix.

It is a general characteristic that a group of such resistors fabricated from the same materials and processed in the same equipment will not usually all have the same electrical resistance value after the last step in normal fabrication-the firing operation. Within the group, resistance value variations as high as 25% from the nominal resistance value will often occur. As a result, normal practice has been to design the cermet electrical resistor to have an initial resistance value which is lower than the resistance value desired, about 70% of the desired value, and to increase the initial resistance value obtained after firing to the desired value, within acceptable limits, of accuracy, by physical removal of cermet resistor material, usually from the upper surface of the resistor, thereby to decrease the thickness of the resistor and hence increase its resistance. This is generally accomplished by an abrasion process. The use of abrasion for this purpose is disadvantageous-it marks the glass surface of the resistor and leaves it vulnerable to subsequent variations in resistance value caused by infiltration of moisture into the resistor and consequent oxidation of the metallic alloy conductive medium contained within the glass matrix.

In accordance with the present invention cermet resistors of a specific type are trimmed by a method which is electrical as distinguished from mechanical, which is more readily suspectible of production used by semi-skilled personnel than is the former abrasive trimming method, and which maintains the physical integrity 3,457,637 Patented July 29, 1969 of the resistive coating and thus eliminates those changes in the resistance value attributable to prior art abrasive trimming methods. The electrical trimming method of the present invention further differs significantly from the abrasive trimming method of the prior art, and from other electrical trimming methods used in the past in connection with different types of resistors, in that the method of the present invention results in a reduction in the resistance value of the unit rather than in an increase of that resistance value. The method of the present invention not only reduces the resistance value of the cermet resistor but also stabilizes that resistance value at a predetermined value against changes which otherwise tend to occur due to aging, electrical operation, and environmental effects.

As has been indicated, the cermet electrical resistor to which this invention is applicable is of a specific known type, one which utilizes a particulate conductive medium contained within a glass matrix. The conductive medium consists of discrete particles of a non-oxidizable, noble metal alloy which has a melting point in excess of the softening point of the glass matrix and hence, in a typical application, in excess of about 1200 C. The noble metal alloys are usually composed only of combinations of gold, platinum, rhodium and iridium. The insulating matrix consists of a glass typically melting between about 500 C. and about 1000 C. Conventionally it comprises silicon dioxide as its major constituent, and additives such as potassium oxide, calcium oxide, aluminum oxide, lead oxide, boron oxide, and the like are employed to modify its melting characteristics and its thermal expansion characteristics, all as is well known in glass-making technology. The specific compositions of the glass matrix and of the conductive particles are known in the art and the details thereof may be varied Within the limits set forth above insofar as the present invention is concerned. Hence such details are not here set forth in extenso. It may be mentioned, however, that the presence of sodium in the glass is generally considered undesirable because of the tendency of that element to migrate when exposed to an electrical potential.

In forming a resistor of the type here involved, the noble metal alloy particles are homogeneously blended with particles of the glass matrix in oppropriate amounts and the mixture is then applied to a desired portion of an appropriate insulating surface, as through the use of a silk screen process, for example. The mixture is then fired by bringing it to the melting temperature of the glass and then cooling it. The resultant resistor consists of a multitude of noble metal alloy particels suspended in the insulating glass matrix. Because the particles do not oxidize and have a melting point above that of the glass matrix, the particles substantially retain their particulate character during the firing operation, and no appreciable sintering, grain growth or plastic flow occurs.

Electrical conduction through such a resistor is believed to occur only through such paths as may be defined by physical contact of the noble metal alloy particles with one another. The degree of contact at any given point will be unpredictable, but because of the very large number of particles present the overall contact characteristics will follow a statistically predictable pattern. Some particles will be touching along major portions of their surfaces, some will scarcely touch, but contiguous particles will provide a continuous path for electrical current from one end of the resistor to the other.

As has been mentioned above, it has not proved possible to manufacture resistors of the type under discussion to a high degree of precision insofar as resistance value is concerned, even when the same cermet composition and firing conditions are used. The prior art abrasive method of tailoring or trimming cermet resistors, heretofore described, is based upon the principle that the internal characteristics of the fired resistor material itself cannot be varied, the trimming therefore being directed to changing the physical dimensions of the resistor. The method of the present invention, however, takes a fundamentally different approach. It does modify the internal characteristics of the fired cermet material by utilizing certain specific characteristics thereofconductivity and nonoxidizability of the particles, random nature of the contact between the particles, high melting point particles within low melting point matrix.

In accordance with the present invention the physical and electrical contact between abutting conductive particles is controllably improved, thus reducing the resistance value and making that value more stable. The method consists of causing a current of appropriate magnitude and duration to flow through the conductive particles, as by applying a voltage across the resistor. The current flow produces heat directly within the conductive particles, those particles therefore being more strongly subjected to the heat than is the surrounding glass matrix. The amount of current and the duration of current flow is so chosen as to cause the contacting portions of abutting conductive particles to fuse t-o one another to desired degree, thereby improving the electrical connection between the adjacent particles and hence reducing the overall resistance value of the resistor. Since the heat produced at any given point is proportional to the resistance thereof, and since the resistance between adjacent particles is inversely proportional to the degree of physical contact therebetween, the greater amounts of heat will inherently be selectively produced at those points throughout the resistor where the resistance is greatest, and particularly at those microscopic points where the noble metal particles are in contact only over comparatively small areas. Hence it is precisely at those points of maximum resistance that the greatest resistance-lowering effect occurs. The extent to which the resistance value of the resistor is reduced is controlled by applying to the resistor only sufiicient voltage (and hence only causing suflicient current to flow therethrough) to fuse together the desired (but unknown) number of noble metal alloy particles to a desired degree.

The electrical voltage required to effect a desired reduction in resistance for a particular resistor of the type here involved is diflicult to predict in advance. Resistor configuration (length, width and thickness), sheet resistivity (determined by the relative proportion of noble metal alloy particles and glass), and the composition of the noble metal alloy are the governing factors. The trimming procedure is best approached empirically, with the voltage generally being applied in short pulses of about 0.5-3 seconds in duration. The resistance value of the resistor may be measured after each pulse, thereby to determine when the desired resistance value has been attained. Reduction in resistance value will continue, within limits, as the total duration of trimming current flow increases; further or more rapid reduction in resistance value can be achieved by increasing the trimming voltage, thereby to cause an increase in the trimming current. In typical production practice the cermet resistor is initially designed to have an electrical resistance value from 30-250% higher than that desired, after which the trimming procedure is carried out to reduce the resistance to its desired value.

After the desired resistance value has been attained, it may be reliably stabilized, without change in its value, if the resistor is thereafter subjected to a final voltage pulse having a magnitude from 50-90% of the highest voltage used in the prior trimming steps and for a period of time of about 15-60 seconds. The stabilizing voltage should also be higher than the voltage to which the resistor is designed to be subjected in use. The reason why this stabilization efi'ect occurs is not known, and it must therefore be considered as purely empirical.

By way of example, a 100,000 ohm resistor may be produced by forming a resistor strip having a width of 0.015 inch, a length of 0.030 inch and a thickness of 0.0015 inch from fired resistor material having a sheet resistivity of about 75,000 ohms per square inch. The untrimmed resistor thus produced will have a resistance value of about 150,000 ohms. That resistor is trimmed by applying a trimming voltage thereto which is increased to 275 volts in pulses of about 3 seconds in duration until the desired 100,000 ohm resistance value has been attained. For final stabilization the resistor is then subjected to a voltage of 160 volts for a period of about 15 seconds. The resistor so produced will retain its 100,000 ohm resistance value within a tolerance of 1% when exposed to air tempertures as high as 450 C., and it may be operated at watt of power at 25 C. and at /8 watt of power at C.

It will be seen that the method of the present invention is readily carried out in a substantially foolproof manner even by semi-skilled personnel, it avoids the impairment of the stability characteristics inherent in the original fired cermet materials, it permits the use of a minimal amount of cermet material in producing a desired high resistance value, and it permits the attainment of resistance value accuracies and stabilities which were not formerly known to be achievable in connection with resistors of the type in question.

The explanation here given for why the method of the present invention is efiicacious is believed to be correct. 'It is, however, a theory, and subsequent investigations may indicate that there are additional or different reasons why the method has its characteristic results. The method may therefore be considered as purely empirical, and the commercial significance thereof is entirely independent of the theoretical explanation set forth.

While only a limited number of embodiments of the present invention have been here specifically disclosed, it will be apparent that many variations may be made therein, all within the scope of the instant invention as defined in the following claims.

I claim:

1. The method of reducing and stabilizing the resistance value of a resistor comprising discrete noble metal alloy particles dispersed in a glass matrix, the melting point of said glass being substantially higher than that of said alloy, which method comprises: applying a voltage across said resistor for a limited period of time and thereby causing a trimming current on the order of milliamps to flow through said resistor, said voltage having a value and time duration such as to produce a predetermined decrease of resistance value in said resistor.

2. The method of claim 1, in which, after said predetermined resistance has been produced, said resistor is subjected for a predetermined period of time to a voltage 50-90% of the voltage which produced said trimming current.

3. The method of claim 2, in which said voltage is applied in pulses.

4. The method of claim 2, in which said voltage is applied in pulses having a duration of about 0.5-3 seconds. i

5. In the method of claim 2, applying said voltage 1n pulses having a duration of about 0.5-3 seconds, and monitoring the resistance value of said resistor in the intervals between said pulses.

6. The method of claim 1, in which, after said predetermined resistance has been produced, said resistor is subjected for a predetermined period of time having a duration of about 15-60 seconds to a voltage 50-90% of the voltage which produced said trimming current.

7. The method of claim 6, in which said voltage is applied in pulses.

8. The method of claim 6, in which said voltage is applied in pulses having a duration of about 0.5-3 seconds.

9. In the method of claim 6, applying said voltage in pulses having a duration of about -23 seconds, and monitoring the resistance value of said resistor in the intervals between said pulses.

10. The method of claim 1, in which said voltage is applied in pulses.

11. The method of claim 1, in which said voltage is applied in pulses having a duration of about 0.5-3 seconds.

12. In the method of claim 6, applying said voltage in pulses having a duration of about 0.5-3 seconds, and monitoring the resistance value of said resistor in the intervals between said pulses.

13. The method of reducing and stabilizing the resistance value of a resistor comprising discrete noble metal alloy particles dispersed in a glass matrix, the melting point of said glass being substantially higher than that of said alloy, which method comprises: subjecting said noble metal alloy particles to the action of heat to a greater degree than said glass matrix is thus subjected and to a degree suflicient to cause abutting noble metal alloy particle surfaces to fuse to one another to a pre determined degree, thereby to decrease said resistance value to a predetermined value.

14. The method of claim 13, in which, after said fusion to a predetermined degree has been attained, said particles are subjected, for a predetermined period of time, to the action of heat to a greater degree than said glass matrix and to a degree between about 25-80% of the maximum degree utilized to produce said predetermined degree of fusion.

15. The method of claim 14, in which said heat producing said fusing to said predetermined degree is applied to said particles in pulses.

16. The method of claim 14, in which said heat producing said fusing to said predetermined degree is applied to said particles in pulses having a duration of about 0.5-3 seconds.

17. In the method of claim 14, subjecting said particles to said heat producing said fusing to said predetermined degree in pulses having a duration of about 0.5-3 seconds, and measuring the resistance value of said resistor in the intervals between said pulses.

18. The method of claim 13, in which, after said fusion to a predetermined degree has been attained, said particles are subjected, for a predetermined period of time having a duration of about 15-60 seconds, to the action of heat to a greater degree than said glass matrix and to a degree between about 25-80% of the maximum degree utilized to produce said predetermined degree of fusion.

19. The method of claim 18, in which said heat producing said fusing to said predetermined degree is applied to said particles in pulses.

20. The method of claim 18, in which said heat producing said fusing to said predetermined degree is applied to said particles in pulses having a duration of about 0.5-3 seconds.

21. In the method of claim 18, subjecting said particles to said heat producing said fusing to said predetermined degree in pulses having a duration of about 0.5-3 seconds, and measuring the resistance value of said resistor in the intervals between said pulses.

22. The method of claim 13, in which said heat producing said fusing to said predetermined degree is applied to said particles in pulses.

23. The method of claim 13, in which said heat producing said fusing to said predetermined degree is applied to said particles in pulses having a duration of about 0.5-3 seconds.

24. In the method of claim 13, subjecting said particles to said heat producing said fusing to said predetermined degree in pulses having a duration of about 0.5-3 seconds, and measuring the resistance value of said resistor in the intervals between said pulses.

References Cited UNITED STATES PATENTS 3,261,082 7/1966 Maissel et al. 29-620 3,308,528 3/1967 Bullard et al. 29-620 3,371,411 3/ 1968 Russell 29-620 X 3,388,461 6/1968 Lins 29-620 X JOHN F. CAMPBELL, Primary Examiner J. L. CLINE, Assistant Examiner 

